The present invention relates to a radio packet communication receiver system using orthogonal multi-carrier modulation system, in particular, relates to such a receiver system which detects reference received timing accurately under severe environment of large carrier frequency offset between a transmit side and a receiver side, and/or multi-path propagation, and/or presence of thermal noise.
An orthogonal multi-carrier modulation system transmits high rate signal by using a plurality of sub-carriers each satisfying orthogonal relation, and called OFDM (Orthogonal Frequency Division Multiplexing) system. An OFDM signal can be modulated or demodulated by using (Inverse) Fast Fourier Transform ((I)FFT) circuit. An OFDM system has a feature that an inter-symbol interference is avoided if multi-path delay is within a guard interval, which is added to each OFDM signal so that it carries an OFDM signal cyclically. Therefore, an OFDM system is excellent to combat multi-path environment in high rate transmission.
Because of the excellent multi-path immunity, a radio computer network communication (radio LAN) is expected to use an OFDM system. In a computer network communication, data length is not fixed, and a packet signal in which received timing is indefinite is used. In radio transmission of such a packet signal, burst reception process is essential for independent synchronization of each received packet, including determination of reference timing of a received symbol.
A technical standard IEEE802.11a is one of the radio LAN""s using OFDM modulation system. That technical standard can satisfy high rate transmission higher than 20 Mbit/sec.
FIG. 11A shows a packet format which uses an OFDM modulation scheme, having a preamble for synchronization, a preamble for channel estimation, and a plurality of OFDM signals OFDM1, OFDM2, OFDM3 , , , . An preamble for channel estimation, and each OFDM signals comprises a data following a guard interval (GI2, GI). A guard interval has a repetition of waveform at the end of the following OFDM data cyclically. A preamble for synchronization comprises a plurality of repetitive known data patterns called a short interval (in the embodiment, 10 short intervals t1 through t10 are provided). A preamble for channel estimation comprises known data patterns T1 and T2 following a guard interval G12, for demodulating sub-carriers (channel demodulation for coherent detection). Each OFDM signal is used for carrying transmission data. At least one of the OFDM signals (for instance OFDM1) is used for showing a property (modulation scheme, transmission rate, length of a packet et al) of succeeding OFDM signals.
A preamble for synchronization (t1 through t10) is used for synchronizing receiver carrier frequency with transmitter carrier frequency, and defining a reference timing of a receiver.
The present invention provides accurately a reference timing of a preamble for synchronization. The reference timing is at a rear end of the short timing t10. By using the accurate reference timing of the preamble for synchronization, a carrier frequency of a receiver is synchronized with a carrier frequency of a transmitter, a guard interval is removed from each OFDM signal for Fourier transformation, and coherent detection of each OFDM signal is carried out for demodulating each sub-carriers.
FIG. 11B shows a block diagram of a prior art of an OFDM packet communication receiver. A received signal R is a radio packet signal shown in FIG. 11A. A received signal R is applied to a correlator 301 which has coefficient of a known pattern (short preamble) of a preamble for synchronization, so that the correlator 301 provides a high level of correlation output signal B when the known pattern (short preamble) is received.
The correlation output signal B is high and has period of the short preamble when a preamble for synchronization is received, and said signal B is low when other signals are received. The correlation output signal B is applied to a timing decision circuit 303, which recognizes the presence of a preamble for synchronization when the correlation output signal B exceeds a threshold level, and recognizes the end of the preamble for synchronization when the correlation output signal B is decreased lower than another threshold level after repetition period of the preamble for synchronization. Then, a reference timing signal D, or a symbol timing signal D is obtained. As a received signal R and an output of the correlator 301 are complex signal having a real part and an imaginary part, the correlation output signal must be converted to a scalar signal when it is applied to the timing decision circuit 303.
A timing decision circuit 303 in a prior art has a delay circuit 37 having delay time T (T=t1=t2=- - - =t10) coupled with a correlation output signal B, a first threshold circuit 39 coupled with an output of said delay circuit 37, a second threshold circuit 40 coupled with a correlation output signal B, and a logic circuit 43 coupled with an output of two threshold circuits to provide a symbol timing signal D when the first threshold circuit 39 shows that an output of the delay circuit 37 is higher than a first threshold level and the second threshold circuit 40 shows that a signal B is lower than a second threshold level.
Thus, the correlation circuit 301 and the timing decision circuit 303 compose a symbol timing detection block 10A. The symbol timing signal D is applied to a frequency offset compensation circuit 20 and a guard interval remove circuit 4. The frequency offset compensation circuit 20 recognizes repetition signals in a received signal R, or a preamble for synchronization, so that it measures carrier frequency offset between a transmit side and a receiver side by measuring phase rotation between repetition waveforms, and compensates said carrier frequency offset .
An output signal A of the carrier frequency offset compensation circuit 20 is applied to a guard interval removal circuit 4, which removes a guard interval GI or GI2 in an OFDM signal in an output A of the frequency offset compensation circuit 20.
After removal of a guard interval, an OFDM symbol E which has no guard interval is applied to a Fourier transform circuit 5 which provides sub-carrier vectors F of each sub-carriers. The sub-carrier vectors F are applied to a coherent detection circuit 6 for coherent detection of each sub-carriers to provide coherent detected signal G. Further, the signal G is applied to a code decision circuit 22 for deciding a code 0 or 1 to provide a received data G2.
By the way, a received signal in radio communication is subject to thermal noise undesirably generated in a receiver amplifier, and/or undesired interference noise. Further, a propagation path is a combination of multi-paths including a direct path and an indirect path reflected by a wall. An OFDM system can provide a high quality transmission even under a multi-path propagation, because of the presence of a guard interval.
However, in an OFDM system in a radio packet communication, a synchronization including decision of a received symbol timing must be established for each packet independently, and the synchronization must be accurate for enjoying to combat multi-path propagation in an OFDM system. Further, a preamble for synchronization in radio packet signal is preferably as short as possible for high rate transmission, and synchronization process is preferably as quick as possible by using a short preamble signal.
A prior burst OFDM receiver has a correlator for recognizing a preamble, and a timing decision circuit for deciding the presence of a preamble in an output of the correlator. However, an accurate decision of a symbol timing would be difficult or a symbol timing would be erroneously decided under large noise environment, and/or multi-path propagation with many delayed waves.
Further, a decided symbol timing is used for compensating carrier frequency offset in a receiver in a prior art. Accordingly, when carrier frequency offset is large between a transmit side and a receiver side, an output of a correlator is decreased, and thus, the decision of a symbol timing would be difficult under multi-path and/or large noise environment.
It is an object, therefore, of the present invention to overcome the disadvantages and limitations of a prior OFDM packet communication receiver system by providing a new and improved OFDM packet communication receiver system.
It is also an object of the present invention to provide an OFDM packet communication receiver system which provide accurate reference received timing even under large carrier frequency offset environment between a transmit side and a receiver side, and/or multi-path propagation environment, and/or thermal noise environment.
The above and other objects are attained by an OFDM packet communication receiver system which receives at least a preamble for synchronization having a plurality of repetitive known short preambles followed by at least one OFDM signal having a guard interval followed by a data signal modulated with multi-carrier modulation comprising; a carrier frequency offset compensation means for compensating carrier frequency offset between a transmit side and a receiver side by using said preamble for synchronization; symbol timing detection means for determining a reference timing of said preamble for synchronization; guard interval removal means for removing a guard interval of an OFDM signal by using the determined reference timing; Fourier Transform means for Fourier transformation of a data signal which is obtained by removing a guard interval from an OFDM signal to provide a received vector of a sub-carrier; sub-carrier demodulation means for coherent detection of a sub-carrier obtained in said Fourier transform means; code decision means for deciding a code carried in a demodulated sub-carrier; wherein said symbol timing detection means comprises; a correlator for providing a correlation between said short preamble and a known pattern in a vector form; a correlation output filter for filter process of an output of said correlator to remove noise, and conversion of a signal into scalar form; a timing decision means for determining said reference timing which indicated end of said preamble for synchronization by comparing an output of said correlation output filter related to said short preamble with a predetermined threshold level.
Preferably, said correlation output filter comprises; a complex filter having an impulse response in every repetition periods of said short preamble; a scalar conversion means for converting an output of said complex filter into scalar form; and a scalar filter for integrating an output of said scalar conversion means for a predetermined time.
Preferably, said correlation output filter comprises; a complex filter having an impulse response in every repetition periods of said short preamble; and a scalar conversion means for converting an output of said complex filter into scalar form.
Preferably, said correlation output filter comprises; a scalar conversion means for converting an output of said correlator into scalar form; and a scalar filter for integrating an output of said scalar conversion means for a predetermined time.
Preferably, said timing decision means comprises; first detection means for detecting that an output of said correlator exceeds a predetermined threshold level a plurality of times in every repetition periods of said short preamble; and second detection means for detecting, after first detection by said first detection means, that an output of said correlator after one repetition period of said first detection decreases more than a predetermined ratio as compared with a level of said first detection.
Preferably, said timing decision means comprises; first detection means for detecting that an output of said correlator exceeds a first threshold level a plurality of times in every repetition periods of said short preamble; and second detection means for detecting that an output of said correlator decreases lower than a second threshold level after one repetition period of a first detection by said first detection means.
Preferably, said timing decision means comprises; first detection means for detecting that an output of said correlator exceeds a first threshold level a plurality of times in every repetition periods of said short preamble; second detection means for detecting, after first detection by said first detection means, that an output of said correlator after one repetition period of said first detection decreases more than a predetermined ratio as compared with a level of said first detection; third detection means for detecting that an output of a second correlator which detects a second preamble; and means for confirming that said second detection means and said third detection means provide detection outputs simultaneously.
Preferably, said timing decision means comprises; first detection means for detecting that an output of said correlator exceeds a first threshold level a plurality of times in every repetition periods of said short preamble; second detection means for detecting, after first detection by said first detection means, that an output of said correlator after one repetition period of said first detection is lower than a second threshold level; third detection means for detecting that an output of a second correlator which detects a second preamble exceeds a third threshold; and means for confirming that said second detection means and said third detection means provide detection outputs simultaneously.
Preferably, said frequency offset compensation means comprises; means for compensating carrier frequency offset between a repetition period of a short preamble between a transmit side and a receiver side upon each receipt of said short preamble; hold means for holding carrier frequency offset relating to a predetermined short preamble; and means for carrying out frequency offset compensation according to a content of said hold means during reception of an OFDM signal after a preamble for synchronization, so that frequency compensated signal by said frequency offset compensation means is applied to said timing detection means.
Preferably, said frequency offset compensation means comprises; delay means for delaying a received signal for a predetermined time; complex multiplier for providing conjugate complex multiplication of said received signal and an output of said delay means; a moving average circuit for providing moving average of an output of said complex multiplier; an inverse tangent circuit for providing phase value of an output of said moving average circuit; a hold circuit receiving an output of said inverse tangent circuit, and having a control input for switching an output of said hold circuit between a first operation phase that said output of said hold circuit is the same as the input to said hold circuit and a second operation phase that said output of said hold circuit is a content of said hold circuit which holds an input signal of said hold circuit at a predetermined time; a phase compensation calculate circuit for integrating an output of said hold circuit for generating compensation value of phase rotation caused by carrier frequency offset ; and a compensation circuit for complex multiplication of said received signal and an output of said phase compensation calculate circuit to compensate carrier frequency offset in said received signal; and said timing decision means comprises supply means for supplying switching signal to said control input of said hold circuit; said supply means makes said hold circuit output an input to the hold circuit as it is when the receiver system is waiting or receiving a preamble for synchronization, and makes said hold circuit output a content of said hold circuit which holds an output of said inverse tangent circuit at the time of the last short preamble when the receiver system is receiving an OFDM signal after preamble for synchronization.
Still preferably, an input of said timing detection means is supplied by an output of said frequency offset compensation means.